This invention relates to an apparatus for measuring irregularities on a plane, and more particularly to an apparatus for measuring the distribution of irregularities on a mirror surface by optically examining irregularities occurring in part of said surface or strains appearing in the whole thereof.
Where a semiconductor integrated circuit is manufactured, the flatness of the surface of a silicon wafer generally has a close relationship with the reliability of the circuit. The measurement of irregularities on a mirror surface of, for example, a wafer has been carried out by an optical method like that based on interference fringes. This method is to observe interference fringes produced by different paths of light beams reflected from a mirror surface and also from a glass plate placed thereon. If the mirror surface is strained, then reflections from the respective parts of the mirror surface will vary in direction and in the length of light path, resulting in irregular interference fringes. Therefore, the distribution of irregularities on the mirror surface can be measured by observing the irregular interference fringes. The above-mentioned method can indeed measure extensive strains over the whole of a mirror surface, but fails to find local irregularities. Detection of local irregularities has to be carried out by a time-consuming process of analyzing an interference pattern made into a readily observable form.
Other known optical methods include the multidivision method (Supplement to the Journal of the Japan Society of Applied Physics, vol. 44, 1975, P. 261) which comprises the steps of dividing a laser beam by a plane lattice and determining the distribution of irregulaties on a mirror surface from a pattern of divided laser beams reflected from the mirror surface. Another known method is the MOIRE' infringement method which comprises the steps of superimposing two plane lattices having the same lattice constant with their relative positions displaced through a slight angle and observing the resultant MOIRE' infringements. However, all the prior art methods fail to visually measure local irregularities on a mirror surface, though they are capable of examining extensive strains over the whole of said mirror surface.
Further, there has been developed a method like a photomicrometer which chiefly examines local irregularities on a mirror surface conversely from the above-mentioned prior art methods. In that method the photomicrometer is used to attempt to examine strains occurring all over a mirror surface. Then observation spots spread over the entire mirror surface have to be scanned, thus consuming a considerable length of time.
During manufacture of an integrated circuit, an impurity is diffused in a silicon wafer in a diffusion furnace. Therefore, it is preferred that measurement of irregularities on the mirror surface of the silicon wafer be carried out while the wafer is placed in the diffusion furnace. A mechanical method like a contact method presents difficulties in attaining the above-mentioned object.